The major shrimp allergen has been isolated and identified as a tropomyocin based on a comparison of amino acid sequences of different peptides generated by proteolysis of the parent molecule. Dot blot and competitive ELISA inhibition using sera of shrimp sensitive patients revealed that antigenic as well as allergenic activities were associated with two peptides. These peptides have been sequenced. These two peptides are similar between cross-reacting Crustaceae, but vary between tropomyosin from phylogenetically distinct species. Synthetic peptides corresponding to two IgE binding B-cell epitopes have comparable allergenicity to native peptides. Peripheral blood mononuclear cells from patients with allergic eosinophilic gastroenteritis (AEG) over-produced IL-4 and IL-5 with mitogenic stimulation. IL-5 synthesis by CD4+ T lymphocytes occurred in the absence of mitogen. Gamma interferon mRNA was not detected by reverse transcription/ polymerase chain reaction in gut mucosal biopsies from patients with AEG, but was present in controls.